1. Field of the Invention
The present invention relates to air conditioning systems and, more particularly, to a variable air volume air conditioning system.
2. Description of the Prior Art
Heretofore, most commercially acceptable air conditioning systems have been of the multi-zone variety wherein a fixed volume of air was delivered to the conditioned zone and the temperature of the delivered air was varied depending upon the load requirements of the zones. While the multi-zone type of system provided excellent temperature control, it proved to be very inefficient since it mixed heated air and cooled air to attain the desired temperature. The system also delivered a fixed volume of air so that the power required for delivering the air was also fixed. Thus, the multi-zone type of system proved to be inefficient and uneconomical, especially with the current high cost of energy.
Variable air volume systems provide conditioned air at a fixed temperature and merely vary the amount of air delivered to the space in accordance with the cooling requirements of the space. Thus, the variable air volume systems result in a savings in fan horsepower and also in mechanical cooling since only the amount of air required for cooling is actually cooled and delivered. Significant technical problems have been encountered with variable air volume systems since as the cooling load diminishes, the dampers to the spaces being cooled close and result in a substantial increase in duct pressure. The pressure increase results in increased air velocity, which causes obnoxious noise and can damage the over-pressured air ducts and controls. Thus, a means for relieving the duct over-pressure in the variable air volume systems was required.
One solution to the over-pressure problem in variable air volume systems was to provide a bypass duct for shunting the excess air back to the input of the system. This solution did not provide for any savings in fan horsepower since the same volume of air was handled by the blowers; therefore, one of the advantages of a variable air volume system could not be realized. Another solution to the over-pressure problem was to use dampers at the outlet of the air conditioning unit to reduce the volume of air supplied to the distribution ducts and thereby controlling the duct pressure. The problem encountered in this type of installation was that the entire air conditioning unit was under a very substantial pressure that was detrimental to the air conditioning unit itself, and also substantial pressure drops were experienced across the outlet dampers.
Another solution to the over-pressure problem has been to use an oversized fan having backward-inclined blades with inlet vanes that restrict the inlet open and give the entering air a pre-rotation that reduces the blower capacity depending upon the position of the inlet vanes. This type of installation has been successful in central systems but due to the use of the backward-inclined type of oversized blower, is too expensive for rooftop installations. The backward-inclined type of blade gives good performance where high pressure low velocity type installations are required, such as in high-rise buildings where long duct lengths are standard.
The lack of general acceptance of the prior art variable air volume devices is in part due to the fact that they are not readily adaptable to an economizer-type mode of operation. Thus, the savings in mechanical refrigeration costs associated with economizer operation could not be realized.
Another interesting problem associated with prior art variable air volume systems is that the pressure exerted on the conditioned space would vary depending upon the volume of air delivered thereto. When a large volume of conditioned air was delivered to the space, the pressure within the space would increase and as a result, doors would not close properly since the air pressure would hold the door in a partially open position. At low air delivery volumes, the pressure in the room would be reduced so that difficulty was experienced in drawing the return air back to the conditioning unit. This would be especially true during an economizer mode of operation where much of the room air had to be exhausted. Attempts were made to provide an exhaust fan for removing the room air to compensate for the fresh air being supplied to the room; however, the difference in volume of air being exhausted from the room and that being supplied to the room during different cooling loads would again cause pressure variations in the room. When more air was exhausted than was supplied, a vacuum developed which made it difficult to open doors and also created down-drafts in chimneys which drew flue gas into the building.